Feature: Towns don't build parallel, redundant bridges

src/town_cmd.cpp

@@ -1160,6 +1160,27 @@ static bool CanRoadContinueIntoNextTile(const Town *t, const TileIndex tile, con
 	return Command<CMD_BUILD_ROAD>::Do(DC_AUTO | DC_NO_WATER, next_tile, rcmd, rt, DRD_NONE, t->index).Succeeded();
 }
 
+/**
+ * CircularTileSearch proc which checks for a nearby parallel bridge to avoid building redundant bridges.
+ * @param tile The tile to search.
+ * @param user_data Reference to the valid direction of the proposed bridge.
+ * @return true if another bridge exists, else false.
+ */
+static bool RedundantBridgeExistsNearby(TileIndex tile, void *user_data)
+{
+	/* Don't look into the void. */
+	if (!IsValidTile(tile)) return false;
+
+	/* Only consider bridge head tiles. */
+	if (!IsBridgeTile(tile)) return false;
+
+	/* Only consider road bridges. */
+	if (GetTunnelBridgeTransportType(tile) != TRANSPORT_ROAD) return false;
+
+	/* If the bridge is facing the same direction as the proposed bridge, we've found a redundant bridge. */
+	return (GetTileSlope(tile) & InclinedSlope(ReverseDiagDir(*(DiagDirection *)user_data)));
+}
+
 /**
  * Grows the town with a bridge.
  *  At first we check if a bridge is reasonable.
@@ -1221,6 +1242,11 @@ static bool GrowTownWithBridge(const Town *t, const TileIndex tile, const DiagDi
 	/* Make sure the road can be continued past the bridge. At this point, bridge_tile holds the end tile of the bridge. */
 	if (!CanRoadContinueIntoNextTile(t, bridge_tile, bridge_dir)) return false;
 
+	/* If another parallel bridge exists nearby, this one would be redundant and shouldn't be built. We don't care about flat bridges. */
+	TileIndex search = tile;
+	DiagDirection direction_to_match = bridge_dir;
+	if (slope != SLOPE_FLAT && CircularTileSearch(&search, bridge_length, 0, 0, RedundantBridgeExistsNearby, &direction_to_match)) return false;
+
 	for (uint8 times = 0; times <= 22; times++) {
 		byte bridge_type = RandomRange(MAX_BRIDGES - 1);